The present invention relates to a radio pager and, more particularly, to a radio pager having an automatic area switching capability.
Two kinds of automatic area switching schemes are available with a radio pager having the above capability. A first scheme is to sequentially switch a receipt frequency and set up a frequency at which a synchronizing signal has been received first. A second scheme is to sequentially switch a receipt frequency in order to detect a receipt field strength, and set up a frequency at which a synchronizing signal has been received with a field strength above a preselected value. Japanese Patent Laid-Open Publication No. 3-27631 teaches a third scheme consisting in comparing a received and digitized signal and its regular timing waveform, counting the points of regular transition of the signal in order to determine a frequency at which the field strength is highest, and then setting up such a frequency. The first to third conventional schemes have the following problems left unsolved.
With the first scheme, it is likely that area switching is set at a receipt frequency whose electric strength is low. As a result, automatic area switching must be repeated frequently. This reduces the life of a battery mounted on the radio pager and is apt to omit an incoming call during area switching. This problem stems from the fact that the area switching of the radio pager usually occurs in a region where different service areas overlap each other, i.e., a plurality of frequency signals which the radio pager can receive exist together. With the second scheme, it is difficult to detect electric fields because automatic area switching usually occurs at the peripheral region of a service area where only frequency signals of low field strengths are available. Further, the third scheme is not practicable without resorting to an additional circuit for detecting electric fields, obstructing the miniaturization of the radio pager.